Long-term settlement of tunnel caused by shield tunneling in peaty soil

In order to study the long-term settlement of tunnel caused by shield tunneling in peaty soil, the surrounding soil of tunnel is regarded as continuum, homogeneous and isotropic saturated viscoelastic medium. The rheological properties of peaty soil are described a five-component model. Based on the Terzaghi-Rendulic theory of two-dimensional consolidation, the governing equation for dissipation of the excess pore water pressure are established when the lining is impermeable. The equations are solved using the mathematical methods of separating variables, conformal mapping, Laplace transform and inverse transform, and the analytical solution of the dissipation of excess pore water pressure is obtained. Finally, the vertical strain of soil is integrated to get the formula for long-term settlement of tunnel. The variation laws of the disspation of excess pore water pressure and long-term settlement of tunnel in peaty soil are analyzed through an example. The results show that in the initial stage, the dissipating speed of the excess pore water pressure is faster than that in soft clay in Shanghai, and then it becomes slow and steady. The lasting time of long-term settlement is longer and the settlement amount is larger in the peaty soil. The tunnel settlement tends to be stable in 900 days, and the cumulative settlement is almost up to 150 mm. In addition, the rheological properties of peaty soil in Kunming are significant; if the excess pore water pressure is dissipated by 90% as the completion time of the primary consolidation settlement, the secondary consolidation settlement accounting for about 36% of the total settlement of tunnel is an important part of the long-term settlement of tunnel.